This invention relates to a discharge lamp lighting device employing a booster circuit for starting and/or reigniting discharge lamp means. More particularly the invention relates to filament heating circuit arrangements used in hot cathode discharge lamp lighting device suitable for home use.
In recent years various types of discharge lamp lighting devices have been developed for using energy in an optimum manner to save energy. In such devices an oscillation circuit or booster is used for starting and/or reigniting a discharge lamp, as disclosed in U.S. Pat. Nos. 3,665,243; 3,753,073; 3,758,818; 3,866,088; 3,942,069; 4,081,718; 4,145,638 for a starter, and U.S. Pat. No. 4,079,292. Reference is also made to my U.S. Pat. No. 4,306,177 granted on Dec. 15, 1981; and U.S. Pat. No. 4,238,708, granted on Dec. 9, 1980 referring to discharge lamp lighting devices.
The following explanations shall facilitate the understanding of the functions and problems involved in the initial igniting and subsequent reigniting involved in the so called "every half cycle ignited system" as disclosed in U.S. Pat. No. 4,079,292. FIG. 1 shows a conventional lighting device for discharge lamps comprising an a.c. power source 1, a current limiting ballast choke 2 with primary and secondary windings 21,22, a discharge lamp 3 with filaments 31,32 connected in series with the source 1 through the primary winding 21 of the choke 2, and a high frequency and high voltage generating circuit 4 for generating a high frequency and high voltage output. The high voltage circuit 4 is connected to the lamp 3 at the side opposite the source through the secondary windng 22, and comprises an oscillation circuit 5 including an oscillation capacitor 51 and a series circuit of a thyristor 52 and a non-linear inductor 53, and an intermittent oscillation capacitor 6 connected in series with the oscillation circuit 5. Besides, a circuit using gated thyristors such as a triac, or a circuit of the inverter type may be substituted for the high voltage circuit 4 as long as the circuit generates a high frequency oscillation output intermittently.
In the operation, by switching on the power source 1, the source voltage is supplied to the discharge lamp 3 through the primary winding 21 of the ballast choke 2, and supplied to the high voltage circuit 4 through the primary winding 21, the filaments 31, 32 and the secondary winding 22. As for the high voltage circuit 4, the source voltage is supplied to the thyristor 52 through the intermittent oscillation capacitor 6, so as to break-over the thyristor 52. Thus, the oscillation circuit 5 begins its oscillating operation to generate a high frequency oscillation output. The oscillating operation will continue if the intermittent oscillation capacitor 6 is removed. However, since the capacitor 6 is gradually charged up due to the oscillating operation of the oscillation circuit 5 and the terminal voltage of the capacitor 6 cancels the source voltage, the thyristor 52 becomes non-conductive after a definite time, whereby the high voltage circuit 4 generates the oscillation intermittently with an initial phase of a steeply increasing source voltage in every half cycle. Hence, the high voltage circuit 4 generates an intermittent oscillation output in every fixed phase or rather in every half cycle of the a.c. source voltage.
The oscillation output voltage is divided by the primary winding 21 and secondary winding 22 of the ballast choke 2, and terminal voltage of the primary winding 21 with superposing the source voltage is supplied to the discharge lamp 3. At the same time, the input current of the high tension circuit 4 flows through a closed circuit comprising the elements 1-21-31-22-4-32-1 so as to preheat the filaments 31, 32. After the filaments 31, 32 are sufficiently preheated, the discharge lamp 3 is triggered by the oscillation output of the high voltage circuit 4 and starts its initial ignition. When the lamp is operated, an appearing period of input current becomes shorter than that of the starting time for preheating due to the impedance change of the ballast choke 2 caused by the flowing lamp current. To reduce the preheating current during normal lamp operation the oscillation operation of the circuit 4 and the preheating operation of the filaments 31, 32 are stopped. Thus, the discharge lamp 3 maintains its lit state by the reignition due to the output of the high tension circuit 4 at every half cycle of the power source 1. In such condition, the lamp voltage shows a rectangular waveform with a suspended period corresponded to the period of the intermittent oscillation, the effective value of which becomes somewhat lower compared with that of a conventional discharge lamp lighting system. Since the intermittent input current of the high voltage circuit 4 flows through the ballast choke 2, the waveform of the lamp voltage is somewhat elevated by the effect of the intermittent input current. The appearing phase of the intermittent input current is kept constant regardless of fluctuation of the source voltage, so that the initial phase of the lamp current is maintained to have a constant phase regardless of the fluctuation of the source voltage. Further the intermittent input current has a negative coefficient characteristic to decrease its value due to an encroachment of a remaining portion of the lamp current waveform upon the occurring period of the next half cycle input current if the lamp current increases due to the increase of the source voltage. For this reason, the fluctuation of the lamp current in the "every half cycle ignited lighting system" is favorably maintained in spite of the reduction of the stabilizing inpedance. Calculations of the accumulated energy and necessary inductance of the ballast choke 2, shown that the energy is about 25% and the inductance is about 20% compared to the respective values of a conventional glow start system. Therefore, in the "every half cycle ignited lighting system" the ballast choke 2 may be minimized in accordance with the above ratio. Further, the minimizing ratio becomes more remarkable if it is compared to that of the rapid start system using a step up transformer. Still further, since the phase difference between the source voltage and lamp current is smaller than that of other conventional lighting systems, it becomes possible to omit a power-factor improving capacitor or to use only a capacitor having an extremely small capacitance.
As described above, the "every half cycle ignited lighting system" has remarkable advantages of saving energy and materials. It also makes it possible to use the secondary winding 22 as the ballast choke 2, because of the effect of the improved fluctuation rate due to the input current in spite of the reduction in the number of windings turns of the primary winding 21 of the ballast choke 2. That is, because the impedance of the oscillation circuit 5 in its oscillating operation is small enough to be neglected and the input inpedance of the oscillation circuit 5 in oscillating operation is substantially composed of the capacitive impedance of the ballast choke 2 resulting in a small capacitive impedance of the input part of the oscillation circuit 5, the secondary winding 22 is provided in the ballast choke 2 and is connected across the filaments 31, 32 of the discharge lamp 3 at the side opposite the source whereby it is possible to bring the oscillation phase of the oscillation circuit 5 and the reignition phase of the discharge lamp 3 close to the source voltage phase. Moreover, because the lamp current of the discharge lamp 3 flows through only the primary winding 21 of the ballast choke 2, the ballast choke may be remarkably minimized or reduced in size in comparison with a case wherein the primary winding 21 of the ballast choke 2 is subject to the inductance value of the secondary winding 22.
In the "every half cycle ignited lighting systems" as described above the fluctuation rate is improved by the secondary winding 22 of the ballast choke 2, and the size of the ballast choke 2 can be minimized. However, the filament preheating current tends to be insufficient by reason of the intermittent current for heating the filaments 31, 32 of the discharge lamp 3. To increase the heating current, one may consider arranging the oscillation capacitor 51 in the source side of the circuit between the filaments 31 and 32, but this is difficult for the above arrangement, because the oscillation current of the high voltage circuit 4 is blocked by the secondary winding 22 of the ballast choke 2.
Therefore, a more efficient lighting device supplying a sufficient preheating current for the filaments is required particularly for the home use type of lighting device which requires a larger heating current than that of the commercial use type.